As used herein, the term "angiogenesis" means the generation of new blood vessels into a tissue or organ. Under normal physiological conditions, humans or animals undergo angiogenesis only in very specific restricted situations. For example, angiogenesis is normally observed in wound healing, fetal and embryonal development and formation of the corpus luteum, endometrium and placenta. The term "endothelium" means a thin layer of flat epithelial cells that lines serous cavities, lymph vessels, and blood vessels. The term "endothelial inhibiting activity" means the capability of a molecule to inhibit angiogenesis in general and, for example, to inhibit the growth of bovine capillary endothelial cells in culture in the presence of fibroblast growth factor.
Both controlled and uncontrolled angiogenesis are thought to proceed in a similar manner. Endothelial cells and pericytes, surrounded by a basement membrane, form capillary blood vessels. Angiogenesis begins with the erosion of the basement membrane by enzymes released by endothelial cells and leukocytes. The endothelial cells, which line the lumen of blood vessels, then protrude through the basement membrane. Angiogenic stimulants induce the endothelial cells to migrate through the eroded basement membrane. The migrating cells form a "sprout" off the parent blood vessel, where the endothelial cells undergo mitosis and proliferate. The endothelial sprouts merge with each other to form capillary loops, creating the new blood vessel.
Persistent, unregulated angiogenesis occurs in a multiplicity of disease states, tumor metastasis and abnormal growth by endothelial cells and supports the pathological damage seen in these conditions. The diverse pathological disease states in which unregulated angiogenesis is present have been grouped together as angiogenic dependent or angiogenic associated diseases.
The hypothesis that tumor growth is angiogenesis-dependent was first proposed in 1971. (Folkman J., Tumor angiogenesis: Therapeutic implications., N. Engl. Jour. Med. 285:1182 1186, 1971) In its simplest terms it states: "Once tumor `take` has occurred, every increase in tumor cell population must be preceded by an increase in new capillaries converging on the tumor." Tumor `take` is currently understood to indicate a prevascular phase of tumor growth in which a population of tumor cells occupying a few cubic millimeters volume and not exceeding a few million cells, can survive on existing host microvessels. Expansion of tumor volume beyond this phase requires the induction of new capillary blood vessels. For example, pulmonary micrometastases in the early prevascular phase in mice would be undetectable except by high power microscopy on histological sections.
Examples of the indirect evidence which support this concept include:
(1) The growth rate of tumors implanted in subcutaneous transparent chambers in mice is slow and linear before neovascularization, and rapid and nearly exponential after neovascularization. (Algire G. H., et al. Vascular reactions of normal and malignant tumors in vivo. I. Vascular reactions of mice to wounds and to normal and neoplastic transplants. J. Natl. Cancer Inst. 6:73-85, 1945) PA1 (2) Tumors grown in isolated perfused organs where blood vessels do not proliferate are limited to 1-2 mm.sup.3 but expand rapidly to &gt;1000 times this volume when they are transplanted to mice and become neovascularized. (Folkman J., et al., Tumor behavior in isolated perfused organs: In vitro growth and metastasis of biopsy material in rabbit thyroid and canine intestinal segments. Annals of Surgery 164:491-502, 1966) PA1 (3) Tumor growth in the avascular comea proceeds slowly and at a linear rate, but switches to exponential growth after neovascularization. (Gimbrone, M. A., Jr. et al., Tumor growth and neovascularization: An experimental model using the rabbit cornea. J. Natl. Cancer Institute 52:41-427, 1974) PA1 (4) Tumors suspended in the aqueous fluid of the anterior chamber of the rabbit eye, remain viable, avascular and limited in size to &lt;1 mm.sup.3. Once they are implanted on the iris vascular bed, they become neovascularized and grow rapidly, reaching 16,000 times their original volume within 2 weeks. (Gimbrone M. A. Jr., et al., Tumor dormancy in vivo by prevention of neovascularization. J. Exp. Med. 136:261-276) PA1 (5) When tumors are implanted on the chick embryo chorioallantoic membrane, they grow slowly during an avascular phase of &gt;72 hours, but do not exceed a mean diameter of 0.93+0.29 mm. Rapid tumor expansion occurs within 24 hours after the onset of neovascularization, and by day 7 these vascularized tumors reach a mean diameter of 8.0+2.5 mm. (Knighton D., Avascular and vascular phases of tumor growth in the chick embryo. British J. Cancer, 35:347-356, 1977) PA1 (6) Vascular casts of metastases in the rabbit liver reveal heterogeneity in size of the metastases, but show a relatively uniform cut-off point for the size at which vascularization is present. Tumors are generally avascular up to 1 mm in diameter, but are neovascularized beyond that diameter. (Lien W., et al., The blood supply of experimental liver metastases. II. A microcirculatory study of normal and tumor vessels of the liver with the use of perfused silicone rubber. Surgery 68:334-340, 1970) PA1 (7) In transgenic mice which develop carcinomas in the beta cells of the pancreatic islets, pre-vascular hyperplastic islets are limited in size to &lt;1 mm. At 6-7 weeks of age, 4-10% of the islets become neovascularized, and from these islets arise large vascularized tumors of more than 1000 times the volume of the pre-vascular islets. (Folkman J., et al., Induction of angiogenesis during the transition from hyperplasia to neoplasia. Nature 339:58-61, 1989) PA1 (8) A specific antibody against VEGF (vascular endothelial growth factor) reduces microvessel density and causes "significant or dramatic" inhibition of growth of three human tumors which rely on VEGF as their sole mediator of angiogenesis (in nude mice). The antibody does not inhibit growth of the tumor cells in vitro. (Kim K. J., et al., Inhibition of vascular endothelial growth factor-induced angiogenesis suppresses tumor growth in vivo. Nature 362:841-844, 1993) PA1 (9) Anti-bFGF monoclonal antibody causes 70% inhibition of growth of a mouse tumor which is dependent upon secretion of bFGF as its only mediator of angiogenesis. The antibody does not inhibit growth of the tumor cells in vitro. (Hori A., et al., Suppression of solid tumor growth by immunoneutralizing monoclonal antibody against human basic fibroblast growth factor. Cancer Research, 51:6180-6184, 1991) PA1 (10) Intraperitoneal injection of bFGF enhances growth of a primary tumor and its metastases by stimulating growth of capillary endothelial cells in the tumor. The tumor cells themselves lack receptors for bFGF, and bFGF is not a mitogen for the tumors cells in vitro. (Gross J. L., et al. Modulation of solid tumor growth in vivo by bFGF. Proc. Amer. Assoc. Canc. Res. 31:79, 1990) PA1 (11) A specific angiogenesis inhibitor (AGM-1470) inhibits tumor growth and metastases in vivo, but is much less active in inhibiting tumor cell proliferation in vitro. It inhibits vascular endothelial cell proliferation half-maximally at 4 logs lower concentration than it inhibits tumor cell proliferation. (Ingber D., et al., Angioinhibins: Synthetic analogues of fumagillin which inhibit angiogenesis and suppress tumor growth. Nature, 48:555-557, 1990) PA1 (12) Human retinoblastomas that are metastatic to the vitreous develop into avascular spheroids which are restricted to less than 1 mm.sup.3 despite the fact that they are viable and incorporate .sup.3 H-thymidine (when removed from an enucleated eye and analyzed in vitro). PA1 (13) Carcinoma of the ovary metastasizes to the peritoneal membrane as tiny avascular white seeds (1-3 mm.sup.3). These implants rarely grow larger until one or more of them becomes neovascularized. PA1 (14) Intensity of neovascularization in breast cancer (Weidner N., et al., Tumor angiogenesis correlates with metastasis in invasive breast carcinoma. N. Engl. J. Med. 324:1-8, 1991, and Weidner N., et al., Tumor angiogenesis: A new significant and independent prognostic indicator in early-stage breast carcinoma, J Natl. Cancer Inst. 84:1875-1887, 1992) and in prostate cancer (Weidner N., Carroll P. R., Flax J., Blumenfeld W., Folkman J. Tumor angiogenesis correlates with metastasis in invasive prostate carcinoma. American Journal of Pathology, 143(2):401-409, 1993) correlates highly with risk of future metastasis. PA1 (15) Metastasis from human cutaneous melanoma is rare prior to neovascularization. The onset of neovascularization leads to increased thickness of the lesion and an increasing risk of metastasis. (Srivastava A., et al., The prognostic significance of tumor vascularity in intermediate thickness (0.76-4.0 mm thick) skin melanoma. Amer. J. Pathol. 133:419-423, 1988) PA1 (16) In bladder cancer, the urinary level of an angiogenic peptide, bFGF, is a more sensitive indicator of status and extent of disease than is cytology. (Nguyen M., et al., Elevated levels of an angiogenic peptide, basic fibroblast growth factor, in urine of bladder cancer patients. J. Natl. Cancer Inst. 85:241-242, 1993)
There is also indirect clinical evidence that tumor growth is angiogenesis dependent.
Thus, it is clear that angiogenesis plays a major role in the metastasis of a cancer. If this angiogenic activity could be repressed or eliminated, then the tumor, although present, would not grow. In the disease state, prevention of angiogenesis could avert the damage caused by the invasion of the new microvascular system. Therapies directed at control of the angiogenic processes could lead to the abrogation or mitigation of these diseases.
What is needed therefore is a composition and method which can inhibit the unwanted growth of blood vessels, especially into tumors. The composition should be able to overcome the activity of endogenous growth factors in premetastatic tumors and prevent the formation of the capillaries in the tumors thereby inhibiting the growth of the tumors. The composition, and antibodies specific to the composition, should also be able to modulate the formation of capillaries in other angiogenic processes, such as wound healing and reproduction. Finally, the composition and method for inhibiting angiogenesis should preferably be non-toxic and produce few side effects.